Porous silica is used as a filler for liquid chromatography, a shape selective catalyst, adsorption/separation material for various ions, a flatting agent for paints, etc. In a case where the porous silica is used as a filler for liquid chromatography, the porous silica is required to have a low liquid supply resistance and a high separation efficiency.
The liquid supply resistance can be reduced by increasing the particle size of a porous silica. However, if the particle size of the porous silica is increased, it takes time until substances to be separated are absorbed by the porous silica, then reach a central part of the porous silica and then are released from the porous silica, whereby the separation efficiency deteriorates. On the other hand, if the particle size of the porous silica is decreased in order to improve the separation efficiency, the liquid supply resistance becomes high.
In order to satisfy both low liquid supply resistance (pressure loss suppression) and high separation efficiency, it is conceivable to use a core-shell silica wherein a surface of non-porous silica particle is covered with a shell made of porous silica. The core-shell silica has the non-porous silica core particle at the central part, whereby substances to be separated which is adsorbed on the core-shell silica remain on the shell in the vicinity of the surface. Accordingly, the time until the substances to be separated are adsorbed and then released, is short, and the separation efficiency is high. Further, if the silica-core particle is made to be large, while the thickness of the shell is maintained to be thin, the particle size of the core-shell silica can be large, whereby without decreasing the separation efficiency, the liquid supply resistance can be low.
As the core-shell silica, a core-shell silica obtained by the following method has been known:
A method which comprises a step of neutralizing an alkali silicate aqueous solution and a mineral acid aqueous solution at a pH of from 2 to 10 to produce a gel method amorphous silica, a step of wet pulverizing the gel method amorphous silica, and a step of neutralizing an alkali silicate aqueous solution and a mineral acid aqueous solution at a pH of from 5 to 9 in the presence of the wet pulverized gel method amorphous silica to precipitate sedimentation method amorphous silica particles on the surface of the gel method amorphous silica particles (Patent Document 1).
However, the wet pulverized gel method amorphous silica particles are not spherical but non-specific form particles, and the particle size distribution is broad. Accordingly, the core-shell silica obtained by precipitating sedimentation method amorphous silica particles on the surface of the gel method amorphous silica particles has a low sphericity and a broad particle size distribution. Further, there is a problem that if such a core-shell silica is used as a packing material for liquid chromatography, its packing property to a column for liquid chromatography is poor due to low sphericity.
Further, in the porous shell formed by precipitating sedimentation method amorphous silica particles on the surface of the gel method amorphous silica particles, the uniformity in thickness is low, and if it is used as a packing material for liquid chromatography, there is a problem of low separation efficiency.
PATENT DOCUMENT 1: JP3500305